Method and apparatus for transmitting and receiving control channels by restricting a set of the control channels in a wireless communication system

ABSTRACT

A method and apparatus are provided for transmitting control information in a base station for a wireless communication system. The method includes transmitting, to a terminal, information associated with a number of Control Channel Elements (CCEs) included in control channels, determining a set of control channel candidates based on an IDentifier (ID) of the terminal and the information associated with the number of CCEs; selecting at least one control channel candidate from among the set of control channel candidates; and transmitting the control information to the terminal through the selected at least one control channel candidate.

PRIORITY

This application is a Continuation of U.S. application Ser. No.13/481,044, which was filed in the U.S. Patent and Trademark Office onMay 25, 2012, and is a Continuation of U.S. Pat. No. 8,189,502, whichwas filed in the U.S. Patent and Trademark Office on Apr. 30, 2008, andclaims priority under 35 U.S.C. §119(a) to Korean Patent ApplicationSerial Nos. 2007-42817 and 2007-79269, which were filed in the KoreanIntellectual Property Office on May 2, 2007 and on Aug. 7, 2007,respectively, the disclosures of each of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a wireless communicationsystem, and in particular, to a method and apparatus for transmittingand receiving control channels in an Orthogonal Frequency DivisionMultiple Access (OFDMA) system.

2. Description of the Related Art

Recently, in wireless communication systems, intensive research is beingconducted on Orthogonal Frequency Division Multiplexing (OFDM) andOrthogonal Frequency Division Multiple Access (OFDMA) as a useful schemefor high-speed data transmission in wireless channels.

OFDM, a scheme for transmitting data using multiple carriers, is a typeof Multi-Carrier Modulation (MCM) that converts a serial input symbolstream into parallel symbol streams and modulates each of the parallelsymbol streams with multiple orthogonal subcarriers or subcarrierchannels before transmission.

FIG. 1 is a diagram illustrating a structure of a transmitter in aconventional OFDM system. Referring to FIG. 1, an OFDM transmitterincludes an encoder 101, a modulator 102, a serial-to-parallel converter103, an Inverse Fast Fourier Transform (IFFT) block 104, aparallel-to-serial converter 105, and a Cyclic Prefix (CP) inserter 106.The encoder 101, i.e., a channel encoding block, performs channelencoding on a specific input information bit stream. Generally, aconvolutional encoder, a turbo encoder, or a Low Density Parity Check(LDPC) encoder is used as the encoder 101. The modulator 102 generatesmodulation symbols by performing modulation, such as Quadrature PhaseShift Keying (QPSK), 8-ary Phase Shift Keying (8 PSK), 16-ary QuadratureAmplitude Modulation (16 QAM), 64 QAM, and 256 QAM, on the output of theencoder 101. Although not illustrated in FIG. 1, a rate matching blockfor performing repetition and puncturing can be further included betweenthe encoder 101 and the modulator 102. The serial-to-parallel converter103 serves to convert the serial output of the modulator 102 intoparallel data.

The IFFT block 104 performs an IFFT operation on the output of theserial-to-parallel converter 103. The output of the IFFT block 104 isconverted into serial data by the parallel-to-serial converter 105.Thereafter, the CP inserter 106 inserts a CP code into the output of theparallel-to-serial converter 105.

The Long Term Evolution (LTE) system, which is now under discussion asthe next generation wireless communication system of the UniversalMobile Telecommunication Service (UMTS) system in the 3^(rd) GenerationPartnership Project (3GPP) standardization organization, uses SingleCarrier Frequency Division Multiple Access (SC-FDMA) for the uplink tosolve the high Peak-to-Average Power Ratio (PAPR) problem of OFDMA.SC-FDMA, a type of OFDM, can be realized by adding an FFT block to afront of the IFFT block 104 and pre-coding pre-IFFT data.

FIG. 2 conceptually illustrates resources of a conventional OFDM system.As illustrated in FIG. 2, in OFDM or SC-FDMA, wireless resources areexpressed as a two-dimensional time-frequency array. More specifically,the horizontal axis represents a time domain 201 and the vertical axisrepresents a frequency domain 202. In the time domain 201, 7 OFDMsymbols constitute one slot 204, and two slots constitute one subframe205. Generally, one subframe 205 has the same length as a TransmissionTime Interval (TTI), which is the basic transmission unit.

FIG. 3 illustrates a data transmission and reception procedure between abase station and a terminal in a conventional OFDM system. Referring toFIG. 3, in step 303, a terminal 302 generates a Channel QualityIndicator (CQI) indicating a downlink channel state based on a receivedReference Signal (RS) transmitted by a base station 301, and transmitsthe CQI to the base station 301 in step 304. In this case, the terminal302 can transmit a Channel Sounding Reference Signal (CS/RS) along withthe CQI in order for the base station 301 to identify the uplink channelstate.

Upon receiving the CQI and/or the CS/RS, the base station 301 determinesdownlink or uplink resources it will allocate to the terminal 302through scheduling in step 305, and transmits a scheduling grantindicating the determined downlink/uplink resources to the terminal 302in step 306. The terminal 302 first determines if the scheduling granthas been delivered to the terminal 302 itself If the scheduling granthas been delivered to the terminal 302, the terminal 302 recognizes theallocated downlink/uplink resources indicated by the scheduling grant instep 307, and exchanges data with the base station 301 using theallocated downlink/uplink resources in step 308.

The base station 301 delivers the information necessary for datatransmission/reception to the terminal 302 using a scheduling grant, andthe scheduling grant is transmitted to the terminal 302 through aPhysical Downlink Control Channel (PDCCH). The PDCCH uses some of theresources illustrated in FIG. 2. The base station 301 selects one ormultiple PDCCHs from among a plurality of available PDCCHs, andtransmits the scheduling grant to the terminal 302 through the selectedPDCCH(s).

Because the terminal 302 does not know which channel among the multiplePDCCHs transmitted by the base station 301 is used for the terminal 302,the terminal 302 must monitor all PDCCHs transmitted by the base station301 to determine if there is a PDCCH having a scheduling grant beingtransmitted to the terminal 302. In this case, if the number of PDCCHstransmitted by the base station 301 is great, the terminal 302 mustperform a large number of reception operations to check the schedulinggrant, requiring complicated reception structure and increasing powerwaste of the terminal.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been designed to address at leastthe problems and/or disadvantages in the prior art and to provide atleast the advantages described below.

Accordingly, an aspect of the present invention is to provide a methodand apparatus for transmitting and receiving control channels byrestricting a set of the control channels in a wireless communicationsystem.

Another aspect of the present invention is to provide a method andapparatus for restricting a monitoring set of control channels that aretransmittable to a terminal in a wireless communication system.

In accordance with an aspect of the present invention, a method isprovided for transmitting control information from a base station of awireless communication system. The method includes transmitting, to aterminal, information associated with a number of Control ChannelElements (CCEs) included in control channels; determining a set ofcontrol channel candidates based on an IDentifier (ID) of the terminaland the information associated with the number of CCEs; selecting atleast one control channel candidate from among the set of controlchannel candidates; and transmitting the control information to theterminal through the selected at least one control channel candidate.

In accordance with another aspect of the present invention, a method isprovided for receiving control information in a terminal of a wirelesscommunication system. The method includes receiving, from a basestation, information associated with a number of Control ChannelElements (CCEs) included in control channels; determining a set ofcontrol channel candidates based on an IDentifier (ID) of the terminaland the information associated with the number of CCEs; and monitoringat least one control channel candidate belonging to the set of controlchannel candidates to receive the control information.

In accordance with another aspect of the present invention, an apparatusfor transmitting control information from a base station of a wirelesscommunication system is provided. The apparatus includes a schedulerthat determines information associated with a number of Control ChannelElements (CCEs) included in control channels, determines a set ofcontrol channel candidates based on an IDentifier (ID) of the terminaland the information associated with the number of CCEs, and selects atleast one control channel candidate from among the set of controlchannel candidates; and a transmission unit that transmits the controlinformation to the terminal through the selected at least one controlchannel candidate.

In accordance with another aspect of the present invention, an apparatusfor receiving control information in a terminal of a wirelesscommunication system is provided. The apparatus includes a setdeterminer that determines a set of control channel candidates based onan IDentifier (ID) of the terminal and information associated with anumber of Control Channel Elements (CCEs); and a reception unit thatreceives the information associated with the number of CCEs included incontrol channels from a base station and that receives controlinformation by decoding at least one control channel candidate belongingto the set of control channel candidates.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a diagram illustrating a structure of a transmitter in aconventional OFDM system;

FIG. 2 is a diagram conceptually illustrating resources of aconventional OFDM system;

FIG. 3 is a diagram illustrating a data transmission and receptionprocedure between a base station and a terminal in a conventional OFDMsystem;

FIG. 4 is a diagram illustrating resource mapping between controlchannels according to an embodiment of the present invention;

FIGS. 5A and 5B are diagrams illustrating a variable set of PDCCHs thata terminal monitors depending on a change in all PDCCH candidatesaccording to an embodiment of the present invention;

FIG. 6 is a flowchart illustrating a transmission operation of a basestation according to an embodiment of the present invention;

FIG. 7 is a flowchart illustrating a reception operation of a terminalaccording to an embodiment of the present invention;

FIG. 8 is a block diagram illustrating a transmitter structure of a basestation according to an embodiment of the present invention; and

FIG. 9 is block diagram illustrating a receiver structure of a terminalaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Various embodiments of the present invention will now be described indetail with reference to the annexed drawings. In the followingdescription, a detailed description of known functions andconfigurations incorporated herein has been omitted for clarity andconciseness. Terms used herein are defined based on functions in thepresent invention and may vary according to users, operator intention,or usual practices. Therefore, the definitions of the terms should bemade based on the contents throughout the specification. Although adescription of the present invention will be given herein with referenceto the LTE system, by way of example, the present invention can beapplied to other wireless communication systems to which base stationscheduling is applied, without any modification.

The present invention restricts a set of control channels beingmonitored by a terminal among all available control channelstransmittable by a base station in delivering control information fordata transmission and reception to the terminal in a wirelesscommunication system. That is, the present invention presents a schemefor reducing a procedure that a terminal should perform, by restrictingthe number of control channels that the terminal should receive anddecode. Accordingly, the present invention simplifies a structure of theterminal, and enables the terminal to monitor control channels therebyreducing the battery consumption.

A description of an embodiment of the present invention will be givenherein with reference to a PDCCH, as a control channel, for carrying ascheduling grant for data transmission and reception between a basestation and a terminal. However, it should be noted that the presentinvention is not limited to transmission and reception of PDCCH.

FIG. 4 illustrates resource mapping between control channels accordingto an embodiment of the present invention. Referring to FIG. 4, in theentire frequency bandwidth (or system bandwidth) 401, the smallest timeunit of resources is a slot 403 that includes, for example, 7 OFDMsymbols, and 2 slots constitute one subframe 402. The subframe 402 isthe smallest unit for resource allocation, and generally has the samelength as a Transmission Time Interval (TTI), which is a datatransmission unit. PDCCHs are mapped to several leading OFDM symbols 404(hereinafter referred to as a “control channel resource region”) amongthe multiple OFDM symbols included in the subframe 402, and PhysicalDownlink Shared Channels (PDSCHs), on which packet data are carried, aremapped to the remaining OFDM symbols 405.

PDCCHs for downlink and uplink transmission for multiple terminals existin the control channel resource region 404, and each PDCCH is generatedas described below. That is, a Control Channel Element (CCE) with apredetermined size is used to generate PDCCH, and one PDCCH includes atleast one CCE. That is, a base station transmits control information toa terminal having a good channel state using a PDCCH including one CCE,i.e., by applying a high code rate. However, the base station transmitsthe same sized control information to a terminal having a poor channelstate using a PDCCH including multiple CCEs, i.e., by applying a lowcode rate. Accordingly, even the terminal in the poor channel conditioncan stably receive control information through the PDCCH.

For example, PDCCH candidates 407 are generated using CCE sets including1 CCE 408, 2 CCEs 409, 4 CCEs 410 or 8 CCEs, etc among multiple CCEs 406existing in the entire control channel resource region 404. When thetotal number of CCEs is defined as N, N PDCCH candidates 406 aregenerated when 1 CCE is used for each PDCCH; [N/2] PDCCH candidates aregenerated when 2 CCEs are used for each PDCCH; [N/4] PDCCH candidatesare generated when 4 CCEs are used for each PDCCH; and [N/8] PDCCHcandidates are generated when 8 CCEs are used for each PDCCH. Here, [A]indicates the maximum integer not greater than A.

More specifically, in the example illustrated in FIG. 4, PDCCHs 411 and412 each are allocated to each terminal using one CCE, PDCCH 413 isallocated using two CCEs, and PDCCH 414 is allocated using four CCEs.The PDCCHs 411, 412, 413, and 414 are mapped 415 to the above-statedcontrol channel resource region 404.

The control channel resource region 404, to which multiple PDCCHs aremapped 415, uses a maximum of several OFDM symbols in one subframe. Inthis case, the number of simultaneously used PDCCHs, or the number ofnecessary CCEs, can vary every time, based on the number of terminalscurrently available and channel states of the terminals, and a size ofthe control channel resource region 404 for PDCCHs is variable asillustrates by reference numeral 416. The LTE system can change a size416 of the control channel resource region 404, in which PDCCH(s) areincluded, using periodic information, and particular information(hereinafter referred to as “Category _0 information (Cat0)” or “ControlChannel Format Indicator (CCFI)”) mapped to a predetermined OFDM symbol(for example, the first OFDM symbol) of each subframe, or systeminformation transmitted through a Broadcast Channel (BCH) (hereinafterreferred to as “BCH information”) is used as an example of the periodicinformation.

A terminal monitors only a set of PDCCHs transmittable to the terminalamong all PDCCH candidates transmittable by a base station, withoutreceiving and decoding all of the multiple PDCCH candidates every timeto determine if there is control information scheduled to the terminal.The base station transmits a scheduling grant to the terminal using onlythe restricted set of PDCCHs. Because all the PDCCH candidates can varydepending on the periodic information transmitted by the base station,the set of PDCCHs restricted to the terminal is also subject to changeaccording to the periodic information.

FIGS. 5A and 5B illustrate a variable set of PDCCHs that a terminalmonitors depending on a change in all PDCCH candidates according to anembodiment of the present invention. Referring to FIG. 5A, a basestation can use 4 CCEs 501 to generate PDCCHs. Therefore, the totalnumber of PDCCH candidates 502 is 7. That is, if the possible number ofCCEs, with which PDCCH can be generated, is 1, 2, and 4, four PDCCHcandidates #1, #2, #3, and #4 using one CCE, two PDCCH candidates #5 and#6 using two CCEs, and one PDCCH candidate #7 using four CCEs aregenerated. For example, in FIG. 5A, in the entire PDCCH candidate set502, a first monitoring set 507 indicating PDCCHs that a terminalmonitors is restricted to two PDCCHs 503 and 504 using one CCE, onePDCCH 505 using two CCEs, and one PDCCH 506 using four CCEs. That is,the first monitoring set 507 is restricted to a PDCCH candidate #1 503,a PDCCH candidate #3 504, a PDCCH candidate #5 505, and a PDCCHcandidate #7 506 according to a predetermined rule. Thereafter, the basestation uses one of the PDCCH candidates in the first monitoring set 507to transmit control information to the terminal.

Upon detecting a situation where it should increase the number of PDCCHsfor the terminal, the base station increases the number of CCEs 511 usedfor the PDCCHs to, for example, 8, as illustrated in FIG. 5B, andtransmits periodic information in order to notify the increase to theterminal. Thereafter, the total number of PDCCH candidates 512 increasesto 14, so that PDCCH candidates transmittable to the terminal change asa second monitoring set 517.

More specifically, referring to FIG. 5B, a PDCCH candidate set 512includes 8 PDCCH candidates PDCCH #1-#8 using one CCE, 4 PDCCHcandidates PDCCH #9-#12 using two CCEs, and 2 PDCCH candidates PDCCH #13and #14 using four CCEs. In the PDCCH candidate set 512, the secondmonitoring set 517 for a terminal is restricted to, for example, PDCCHcandidate #2 513, PDCCH candidate #6 514, PDCCH candidate #10 515, andPDCCH candidate #14 516 according to a predetermined rule. Therefore,the base station uses one of the PDCCH candidates in the secondmonitoring set 517 to transmit control information to the terminal.

The number of CCEs constituting each PDCCH can be considered as a rulefor determining a monitoring set of a control channel for the terminal.When the total number of PDCCH candidates 502 is 7, as in FIG. 5A, twoPDCCHs 503 and 504 using one CCE, one PDCCH 505 using two CCEs, and onePDCCH 506 using four CCEs can be restrictively used for the terminal.When the total number of PDCCH candidates 512 is 14, as illustrated inFIG. 5B, two PDCCHs 513 and 514 using one CCE, one PDCCH 515 using twoCCEs, and one PDCCH 516 using four CCEs are restrictively used for theterminal.

Therefore, in restricting the monitoring set of PDCCH candidates for aterminal, the present invention can secure various numbers of CCEs forPDCCHs that the terminal monitors, by applying an independent ruleaccording to the number of CCEs per PDCCH. The application of anindependent rule for the number of CCEs per PDCCH enables maximalinclusion of the different numbers of CCEs per PDCCH in the monitoringset, so that the terminal can use PDCCHs composed of different numbersof CCEs according to the channel condition.

With reference to FIGS. 6 and 7, a description will now be made of atransmission operation of a base station and a reception operation of aterminal according to an embodiment of the present invention. Becauseperiodic information from the base station is subject to change everyscheduling time (or every subframe), establishment of a monitoring setcan also be made every scheduling time.

FIG. 6 is a flowchart illustrating a transmission operation of a basestation according to an embodiment of the present invention. Referringto FIG. 6, a base station performs scheduling on terminals in step 602,and in step 603, determines if it should perform scheduling on aparticular terminal. If it is determined in step 603 that no schedulinghappens on the terminal, i.e., if the terminal is not scheduled, thebase station performs scheduling on other terminals in step 603.However, if it is determined in step 603 that scheduling should happenon the terminal and thus data transmission and/or reception of theterminal is scheduled, the base station sets (determines) a number ofCCEs constituting PDCCH according to the channel state of the terminalin step 604. In step 605, the terminal establishes a monitoring set, ora set of PDCCH candidates that the terminal can receive while satisfyingthe determined number of CCEs per PDCCH.

In establishing the monitoring set, the base station may use at leastone of (i) periodic information Cat0 and BCH that the base stationtransmits to the terminal, (ii) upper layer signaling information givento the terminal, and (iii) a terminal identifier (ID).

In step 606, the base station determines if there is an available PDCCH,unused for other terminals, in the monitoring set. If there is noavailable PDCCH, the base station performs scheduling on other terminalsin step 602, and if there is an available PDCCH, the base stationtransmits a scheduling grant to the terminal using the PDCCH andperforms packet data transmission and/or reception according to thescheduling grant in step 607.

FIG. 7 is a flowchart illustrating a reception operation of a terminalaccording to an embodiment of the present invention. Referring to FIG.7, a terminal establishes a monitoring set, or a set of receivable PDCCHcandidates in step 702. In establishing the monitoring set, the terminalmay use at least one of (i) periodic information Cat0 and BCH that abase station transmits to the terminal, (ii) upper layer signalinginformation given to the terminal, and (iii) a terminal ID.

The terminal monitors and receives each of multiple PDCCHs correspondingto the monitoring set in step 703, and in step 704, determines if thebase station has transmitted a scheduling grant to the terminal throughone of the PDCCHs. More specifically, the terminal determines if thecontrol information received through each of the multiple PDCCHs hasbeen transmitted to the terminal itself, and if control information forthe corresponding terminal is detected from at least one PDCCH theterminal performs packet data transmission and/or reception using thescheduling grant in step 705. However, if no scheduling grant for thecorresponding terminal has been received in step 704, the terminal endsthe reception operation.

FIG. 8 is a block diagram illustrating a transmitter structure of a basestation according to an embodiment of the present invention. Referringto FIG. 8, a monitoring set establisher 803 included in a scheduler 801of a base station establishes a monitoring set, or a set of PDCCHsallowed for each terminal in communication. In establishing themonitoring set, the monitoring set establisher 803 may use at least oneof periodic information 802, such as CatO, and BCH that a base stationtransmits to all terminals, and additional information 804 includingupper layer signaling information given to a terminal, and a terminalID. Further, a CCE count determiner 806 sets (or determines) a number ofCCEs necessary for generating PDCCH of the terminal according to channelinformation 805 determined depending on CQI that the terminal transmitsto the base station, and provides the determined number of CCEs perPDCCH for the monitoring set establisher 803 to use in establishing amonitoring set of PDCCHs.

A data scheduler 807 performs scheduling using the channel information805 of the terminal, and buffer information 809 indicating the amount ofdata that the base station intends to transmit to the terminal. If thereis a need for data transmission and/or reception for the terminal as theterminal is scheduled, a PDCCH selector 808 selects one currentlyavailable PDCCH among the PDCCHs included in the monitoring setestablished for the terminal. A PDCCH generator 811 generates controlinformation, i.e., scheduling grant, according to the PDCCH selected bythe PDCCH selector 808, and inputs it to a multiplexer 820. Here, thePDCCH selector 808 and the PDCCH generator 811 constitute a PDCCHtransmission unit.

A PDSCH generator 810 generates packet data with a predetermined sizeaccording to the scheduling result of the data scheduler 807, and inputsthe packet data to the multiplexer 820. The packet data is generatedwhen downlink data is scheduled, and no packet data is generated whenuplink data is scheduled. The multiplexer (MUX) 820 multiplexes inputinformation and transmits the multiplexed information via a transmissionunit 830.

FIG. 9 is block diagram illustrating a receiver structure of a terminalaccording to an embodiment of the present invention. Referring to FIG.9, a reception unit 901 of a terminal receives a signal from a basestation, and a demultiplexer 902 demultiplexes the received signal intochannel signals. In demultiplexing a PDCCH signal, a monitoring setestablisher 903 uses a monitoring set indicating PDCCHs, which are setto be receivable for the terminal. That is, the monitoring setestablisher 903 establishes a monitoring set including PDCCHs that theterminal monitors according to the same rule based on the sameinformation as that of the monitoring set establisher 803 of the basestation, and controls the demultiplexer 902 to perform demultiplexing onthe PDCCHs belonging to the monitoring set. Because at least one ofperiodic information 904, such as CatO, and BCH that a base stationtransmits to all terminals, and additional information 905 includingupper layer signaling information for a terminal and a terminal ID isused for establishing the monitoring set, the monitoring setsestablished separately by the base station and the terminal include thesame PDCCHs.

The channel signals demultiplexed by the demultiplexer 902 are deliveredto corresponding channel decoders 906 and 907. Particularly, the PDCCHsignals of the monitoring set controlled by the monitoring setestablisher 903 are input to a PDCCH decoder 907. The PDCCH decoder 907determines if a scheduling grant is included in the PDCCH signals, andif it is included, provides the scheduling grant to a datatransmission/reception unit (not shown) in order to perform datatransmission/reception according to the scheduling grant. Here, thedemultiplexer 902 and the PDCCH decoder 907 constitute a PDCCH receptionunit.

Above, a description has been made of an operation and apparatus forrestricting a monitoring set or a set of PDCCHs that a terminalmonitors. A more detailed description will now be made of specificembodiments for establishing a monitoring set for a terminal.

First Embodiment

According to a first embodiment, in establishing a monitoring set ofPDCCHs for a terminal, a base station signals to the terminal a numberof PDCCHs that the terminal should monitor, regardless of a code rate ofeach PDCCH. The code rate of PDCCH is referred to herein as TDCCH MCS',since it is indicated according to a Modulation and Coding Scheme (MCS)level indicating a combination of a modulation scheme and a code ratefor an Adaptive Modulation and Coding (AMC) operation. Here, PDCCH MCScorresponds to the number of CCEs constituting one PDCCH.

A terminal identifies a PDCCH that it should monitor, based on (i) thenumber of PDCCHs for each PDCCH MCS determined according to periodicinformation, such as Cat0 or BCH, (ii) the number of PDCCHs that theterminal should monitor, which is set by upper layer signaling accordingto the terminal, and (iii) a random number generation function that usesa terminal ID or transmission time as its variable. Because a basestation may also determine (i) the number of PDCCHs for each PDCCH MCSdetermined according to periodic information, such as CatO or BCH, (ii)the number of PDCCHs that the terminal should monitor, which is set byupper layer signaling according to the terminal, and (iii) a terminal IDor transmission time, the base station determines PDCCHs that theterminal should monitor, and transmits a scheduling grant using one ofthe determined PDCCHs.

A subframe indicator may be used as transmission time information usedas a variable of the random number generation function. Because anavailable monitoring set is subject to change at every subframe as thetransmission time information is considered, the base station hasflexibility in using PDCCHs. That is, if a PDCCH having a desired PDCCHMCS is not included in a monitoring set in the current subframe as themonitoring set changes at every subframe, there is a possibility thatthe PDCCH having the PDCCH MCS will be included in a monitoring set inthe next subframe.

Second Embodiment

According to a second embodiment, in establishing a monitoring set for aterminal, a base station signals to the terminal a number of PDCCHs thatthe terminal monitors according to a code rate of each PDCCH. Therefore,even though the total number of PDCCHs that the base station can usevaries according to the periodic information, the terminal alwaysmonitors the same number of PDCCHs according to the PDCCH MCS.

The terminal identifies PDCCH that it should monitor for each PDCCH MCS,based on (i) the number of PDCCHs for each PDCCH MCS determinedaccording to periodic information, such as CatO or BCH, (ii) the numberof PDCCHs that the terminal should monitor according to PDCCH MCS whichis set by upper layer signaling according to the terminal, and (iii) arandom number generation function that uses a terminal ID ortransmission time as its variable. Because a base station may alsodetermine (i) the number of PDCCHs for each PDCCH MCS determinedaccording to periodic information, such as CatO or BCH, (ii) the numberof PDCCHs that the terminal should monitor according to PDCCH MCS whichis set by upper layer signaling according to the terminal, and (iii) aterminal ID or transmission time, the base station transmits ascheduling grant using one of the PDCCHs that the terminal can receive.

The second embodiment determines PDCCH that it should monitor using therandom number generation function for each terminal in order to enableone PDCCH to be used with a same probability, if possible, betweenseveral terminals so that several terminals can have flexibility inusing the restricted number of PDCCHs.

Third Embodiment

When a random number generation function is used to determine a PDCCHthat the terminal should monitor, several terminals may receive anddecode the same PDCCH. Therefore, in establishing a monitoring set for aterminal, the third embodiment transmits to the terminal a separatevariable based on PDCCH MCS by upper layer signaling, so the basestation directly determines the PDCCHs that the terminal should monitorfor each PDCCH MCS. For example, the base station signals offset andmodular values for each PDCCH MCS, and sets an i(n)^(th) PDCCHsatisfying Equation (1) as a member of a monitoring set for theterminal.i(n)+offset(n) mod modular(n)=0  (1)

In Equation (1), i(n) denotes an index of PDCCH with PDCCH MCS=n, andits scope is determined according to periodic information, such as CatOor BCH. Further, Offset(n) and modular(n) are variables used for PDCCHMCS=n, and are delivered to the terminal through upper layer signaling.

Because the base station can acquire upper layer signaling informationfor the terminal, it determines PDCCHs that the terminal can receive,using Equation (1), and transmits a scheduling grant using one of thePDCCHs.

Fourth Embodiment

A base station signals an offset value along with the number Num_PDCCHof PDCCHs that a terminal should monitor for each PDCCH MCS, and sets ani(n)^(th) PDCCH satisfying Equation (2) as a member of a monitoring setfor the terminal. In the following, Total_PDCCH (n) denotes the totalnumber of PDCCHs with PDCCH MCS=n, that the base station can transmit,which is determined according to periodic information, such as CatO orBCH.i(n)+offset(n) mod modular(n)=0 modular(n)=[Num_PDCCH (n)/Total_PDCCH(n)]

In Equation (2), i(n) denotes an index of PDCCH with PDCCH MCS=n, andhas a scope ranging from 1 to Total_PDCCH (n). Further, Offset(n) andNum_PDCCH (n) are variables used for PDCCH MCS=n, and are delivered tothe terminal by upper layer signaling.

Because the base station may acquire upper layer signaling informationfor the terminal, it determines PDCCHs that the terminal can receive,using Equation (2), and transmits a scheduling grant using one of thePDCCHs.

Fifth Embodiment

In establishing a monitoring set for a terminal, a fifth embodimentrestricts the number of CCEs that the terminal receives, and usesPDCCH(s) including the restricted number of CCEs as a monitoring set.The number of CCEs that a base station uses in transmitting PDCCH isdetermined according to periodic information, such as Cat0 or BCH, andregardless of the total number of CCEs, the base station notifies thenumber of CCEs that the terminal can receive, to the terminal throughupper layer signaling. The terminal identifies CCEs it can receive,among all the CCEs determined according to the periodic informationsuch, as CatO or BCH, using the number of CCEs that the terminal canreceive, which is determined by upper layer signaling. A random numbergeneration function can be used to determine the CCEs, and indetermining CCEs, consecutive CCEs can be used, the number of which isequal to the number of CCEs that the terminal can receive.

For example, in FIG. 5B, if the terminal is set such that it can receive4 CCEs of CCE5 531, CCE6 532, CCE7 533, and CCE8 534, a set of possiblePDCCHs including CCEs 531-534 is established according thereto. That is,PDCCHs 520, 514, 521, and 522 composed of one CCE, PDCCHs 523 and 524composed of two CCEs, and PDCCH 516 composed of four CCEs are defined asa monitoring set of PDCCHs that the terminal monitors.

Sixth Embodiment

In establishing a monitoring set for a terminal, a sixth embodimentestablishes the PDCCH having the lowest PDCCH MCS as a monitoring set,i.e., only the PDCCH composed of the greatest number of CCEs, among thePDCCHs that the terminal should monitor. Then, regarding CCEsconstituting the PDCCH, the monitoring set for the terminal isdetermined as PDCCHs indicating another PDCCH MCS, which include theCCEs. That is, if the base station notifies PDCCH MCS of a referencePDCCH that the terminal should receive, and the number of PDCCHs havingthe PDCCH MCS, by upper layer signaling, the PDCCHs that the terminalnow monitors is determined based thereon according to a predeterminedrule.

For example, if the reference PDCCH is a PDCCH candidate 516 illustratedin FIG. 5B, the terminal receives four CCEs of CCES 531, CCE6 532, CCE7533, and CCE8 534 for the reference PDCCH 516. Therefore, all otherpossible PDCCHs including the four CCEs 531˜534 are defined as amonitoring set for the terminal. In FIG. 5B, PDCCHs 520, 514, 521, and522 including one CCE and PDCCHs 523 and 524 including two CCEs areincluded in a PDCCH set that the terminal must monitor.

The sixth embodiment can restrict even a particular PDCCH MCS. Forexample, if a channel condition of a terminal is poor, the sixthembodiment makes a restriction such that the terminal should not monitorthe PDCCHs including one CCE. Then the terminal monitors only the threePDCCHs 523, 524, and 516.

Seventh Embodiment

In establishing a monitoring set for a terminal, a seventh embodimentsets a number of CCEs constituting a PDCCH having one PDCCH MCS, i.e.,one PDCCH, among the PDCCHs that the terminal must monitor, andestablishes a monitoring set based thereon. For example, the seventhembodiment establishes a monitoring set that has PDCCH including one CCEas a member, for a terminal in a good channel state, and establishes amonitoring set that has PDCCH including many CCEs as a member, for aterminal in a poor channel state. Although it is assumed in theforegoing other embodiments that multiple PDCCH MCSs are set, the sixthembodiment sets one PDCCH MCS in order to reduce the complexity requiredfor receiving PDCCH(s) for each of various PDCCH MCSs.

A level of the PDCCH MCS is notified separately for each terminalthrough upper layer signaling. The base station sets and notifies PDCCHMCS that uses the less number of CCEs, for a terminal in a betterchannel state according to the average channel state of each terminal,and sets and notifies PDCCH MCS that uses the greater number of CCEs,for a terminal in a worse channel state. If the base stationestablishes, by upper layer signaling, a PDCCH monitoring set that usesa predefined PDCCH MCS for a terminal in the state before PDCCH MCS isset, PDSCH information including upper layer signaling informationindicating PDCCH MCS to be used later can be transmitted through one ofthe PDCCHs in the monitoring set.

For example, if a terminal is set to receive PDCCH including one CCEamong the PDCCH candidates illustrated in FIG. 5B as its channel stateis good, PDCCH candidates #5 520, #6 514, #7 521, and #8 522 among thePDCCH candidates #1-#8 including one CCE are included in a PDCCHmonitoring set.

More specifically, a function of defining PDCCH that the terminal mustmonitor is a random number generation function that has, as its factors,(i) the number of PDCCHs for each PDCCH MCS determined according toperiodic information, such as CatO or BCH, (ii) the number (i.e., alevel of PDCCH MCS) of CCEs used by PDCCHs that the terminal mustmonitor, which is set by upper layer signaling according to theterminal, and (iii) a terminal ID or transmission time.

The foregoing embodiments have presented rules for defining a monitoringset or a set of PDCCHs that the terminal should monitor. In defining themonitoring set of PDCCHs, the embodiments need upper layer signaling,and the upper layer signaling information is transmitted to the terminalthrough a downlink packet. Therefore, the terminal, before it identifiesa set of PDCCHs it should monitor, needs to monitor a PDCCH in order toreceive upper layer signaling information. In this case, the upper layersignaling information is transmitted to the terminal through one of thefollowing methods.

1) A terminal receives PDCCH information used for identifying a set ofPDCCHs it should monitor, using periodic system information (or BCHinformation). In this case, if there is a need for PDCCH to transmit BCHinformation, a base station can separately define PDCCH for, forexample, Dedicated (D)-BCH information in transmitting the D-BCHinformation.

2) A terminal receives PDCCH information used for identifying a set ofPDCCHs it should monitor, using the message information that the basestation provides to the terminal through a Random Access procedure. Inthe RACH procedure, if there is a need for PDCCH in transmitting themessage information over the downlink, PDCCH for the message informationcan be separately defined or can be determined based on the previouslyreceived BCH information.

3) A base station defines a reference set of PDCCHs that a terminalshould always monitor based on a predetermined rule before a set ofPDCCHs the terminal should monitor is determined, and the terminalmonitors the PDCCHs included in the reference set of the terminal.

As is apparent from the foregoing description, in an OFDMA communicationsystem in which a base station transmits a scheduling grant and aterminal receives the scheduling grant, the present invention restrictsa number of control channels that the terminal monitors, therebyreducing the reception complexity of the terminal and avoiding batteryconsumption. In addition, even though the total number of controlchannels varies, the present invention can maintain the number ofcontrol channels that the terminal monitors, to some extent.

While the present invention has been shown and described with referenceto certain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A method for transmitting control informationfrom a base station of a wireless communication system, the methodcomprising: transmitting, to a terminal, information associated with anumber of Control Channel Elements (CCEs) included in control channels;determining a set of control channel candidates based on an IDentifier(ID) of the terminal and the information associated with the number ofCCEs, wherein each control channel candidate includes one, two, four, oreight CCEs; selecting at least one control channel candidate from amongthe set of control channel candidates; and transmitting the controlinformation to the terminal through the selected at least one controlchannel candidate.
 2. The method of claim 1, further comprising the stepof: transmitting, to the terminal, information associated with a numberof Orthogonal Frequency Division Multiplexing (OFDM) symbols carryingthe control channels.
 3. The method of claim 2, wherein the informationassociated with the number of CCEs or the information associated withthe number of OFDM symbols is included in information that isperiodically transmitted to the terminal.
 4. The method of claim 1,wherein the set of control channel candidates includes a plurality ofsubsets, and wherein each of the control channel candidates in a subsetincludes a same number of CCEs.
 5. The method of claim 4, wherein thenumber of CCEs in each of the control channel candidates in one subsetis different from that of another subset.
 6. The method of claim 1,wherein the set of control channel candidates is determined using upperlayer signaling information for the terminal.
 7. The method of claim 1,wherein the set of control channel candidates is determined using arandom number generation function that defines the terminal ID as avariable.
 8. The method of claim 7, wherein the random number generationfunction utilizes transmission time information of a subframe.
 9. Themethod of claim 1, wherein the number of CCEs varies according to achannel environment.
 10. A method for receiving control information in aterminal of a wireless communication system, the method comprising:receiving, from a base station, information associated with a number ofControl Channel Elements (CCEs) included in control channels;determining a set of control channel candidates based on an IDentifier(ID) of the terminal and the information associated with the number ofCCEs, wherein each control channel candidate includes one, two, four, oreight CCEs; and monitoring at least one control channel candidatebelonging to the set of control channel candidates to receive thecontrol information.
 11. The method of claim 10, further comprising thestep of: receiving, from the base station, information associated with anumber of Orthogonal Frequency Division Multiplexing (OFDM) symbolscarrying the control channels.
 12. The method of claim 11, wherein theinformation associated with the number of CCEs or the informationassociated with the number of OFDM symbols is included in informationthat is periodically transmitted from the base station.
 13. The methodof claim 10, wherein the set of control channel candidates includes aplurality of subsets, and wherein each of the control channel candidatesin a subset includes a same number of CCEs.
 14. The method of claim 13,wherein the number of CCEs in each of the control channel candidates inone subset is different from that of another subset.
 15. The method ofclaim 10, wherein the set of control channel candidates is determinedusing upper layer signaling information for the terminal.
 16. The methodof claim 10, wherein the set of control channel candidates is determinedusing a random number generation function that defines the terminal IDas a variable.
 17. The method of claim 16, wherein the random numbergeneration function utilizes transmission time information of asubframe.
 18. The method of claim 10, wherein the number of CCEs variesaccording to a channel environment.
 19. An apparatus for transmittingcontrol information from a base station of a wireless communicationsystem, the apparatus comprising: a scheduler that determinesinformation associated with a number of Control Channel Elements (CCEs)included in control channels, determines a set of control channelcandidates based on an IDentifier (ID) of the terminal and theinformation associated with the number of CCEs, and selects at least onecontrol channel candidate from among the set of control channelcandidates, wherein each control channel candidate includes one, two,four, or eight CCEs; and a transmission unit that transmits the controlinformation to the terminal through the selected at least one controlchannel candidate.
 20. The apparatus of claim 19, wherein the schedulerdetermines information associated with a number of Orthogonal FrequencyDivision Multiplexing (OFDM) symbols carrying the control channels. 21.The apparatus of claim 20, wherein the information associated with thenumber of CCEs or the information associated with the number of OFDMsymbols is included in information that is periodically transmitted tothe terminal.
 22. The apparatus of claim 19, wherein the set of controlchannel candidates includes a plurality of subsets, and wherein each ofthe control channel candidates in a subset includes a same number ofCCEs.
 23. The apparatus of claim 22, wherein the number of CCEs in eachof the control channel candidates in one subset is different from thatof another subset.
 24. The apparatus of claim 19, wherein the set ofcontrol channel candidates is determined using upper layer signalinginformation for the terminal.
 25. The apparatus of claim 19, wherein theset of control channel candidates is determined using a random numbergeneration function that defines the terminal ID as a variable.
 26. Theapparatus of claim 25, wherein the random number generation functionutilizes transmission time information of a subframe.
 27. The apparatusof claim 19, wherein the number of CCEs varies according to a channelenvironment.
 28. An apparatus for receiving control information in aterminal of a wireless communication system, the apparatus comprising: aset determiner that determines a set of control channel candidates basedon an IDentifier (ID) of the terminal and information associated with anumber of Control Channel Elements (CCEs), wherein each control channelcandidate includes one, two, four, or eight CCEs; and a reception unitthat receives the information associated with the number of CCEsincluded in control channels from a base station and that receivescontrol information by decoding at least one control channel candidatebelonging to the set of control channel candidates.
 29. The apparatus ofclaim 28, wherein the reception unit receives information associatedwith a number of Orthogonal Frequency Division Multiplexing (OFDM)symbols carrying the control channels from the base station.
 30. Theapparatus of claim 29, wherein the information associated with thenumber of CCEs or the information associated with the number of OFDMsymbols is included in information that is periodically transmitted fromthe base station.
 31. The apparatus of claim 28, wherein the set ofcontrol channel candidates includes a plurality of subsets, and whereineach of the control channel candidates in a subset includes a samenumber of CCEs.
 32. The apparatus of claim 31, wherein the number ofCCEs in each of the control channel candidates in one subset isdifferent from that of another subset.
 33. The apparatus of claim 28,wherein the set of control channel candidates is determined using upperlayer signaling information for the terminal.
 34. The apparatus of claim28, wherein the set of control channel candidates is determined using arandom number generation function that defines the terminal ID as avariable.
 35. The apparatus of claim 34, wherein the random numbergeneration function utilizes transmission time information of asubframe.
 36. The apparatus of claim 28, wherein the number of CCEsvaries according to a channel environment.